High temperature thermal processes, for example, generation of steam for the production of electricity in power plants utilizing fossil fuels, often create environmentally harmful by-products. These compounds, including nitrogen oxides (NOx) and sulfur dioxide (SO2) must be removed from the flue gases of the high temperature thermal processes before the gases are discharged into the environment, for example before exiting the power plant and contacting the environment.
Desulphurization of the flue gas, for example removal of SO2, may be carried out by applying known methods in which the SO2 produced in the combustion process is oxidized to SO3. This is done prior to exposure of the flue gases to the reduction catalyst. The SO3 may then be absorbed into alkaline solution and removed from the process, usually in the form of gypsum.
The standard for removing nitrogen oxides from flue gases is the selective catalytic reduction (SCR) process, where a reducing agent, typically ammonia, is injected and mixed into the flue gases, and sent through a catalytic reaction chamber where the catalyst facilitates the reduction of NO by the reducing agent to form elemental nitrogen (N2) and water.
One undesired side reaction between the SCR catalyst and the constituents of the flue gas is the conversion of remaining SO2 to SO3. For example, the SO2 remaining in the flue gas may be partially oxidized to SO3, which may then react with water in the flue gas to produce sulfuric acid (H2SO4). Sulfuric acid in the flue gas stream may result in corrosion of steel surfaces, for example surfaces below the dew point of the sulfuric acid, in equipment downstream of the catalyst. In addition, emission of H2SO4 aerosol particles into the atmosphere may also be undesired for environmental reasons.
The catalysts, which in certain embodiments are referred to as DeNOx catalysts, may be constructed of titanium dioxide containing the oxides of transition metals, such as, for example, vanadium, molybdenum, and tungsten, to act as catalytically active components. In specific embodiments, the catalysts may be arranged on plates, in honeycomb fashion, or as a corrugated structure and are generally placed parallel to the direction of the flow of the flue gas. However, during operation of the power plant, the catalyst undergoes a loss of activity and efficiency, for example, due to plugging with fly ash and/or deactivation of the active components by certain compounds contained in the flue gas. Among these compounds are, for example, sodium (Na), potassium (K), arsenic (As), and phosphorous (P) based salts, as well as other compounds.
In addition to these compounds, iron compounds and/or iron salts may physically and chemically bond to the catalyst surface further reducing the performance of the catalyst. Research suggests that iron ions may be the main cause of the enhanced and undesired conversion of SO2 to SO3 during the regular operation of the SCR catalyst in the power plant. Iron contamination may come from a variety of sources, including the fuel burned in the power plant. For example, depending on the origin and age of coal, the natural iron content may range from about 5% to about 8% by weight, relative to the total amount of the mineral components in the coal.
It is generally known that during the regeneration of SCR catalysts, inorganic acids, such as sulfuric acid (H2SO4) and hydrochloric acid (HCl), may be used to clean and restore the catalyst, such as by a soaking step and a neutralizing step. Inorganic acids are typically odorless, which is another advantage to their use. Sulfuric acid, in particular, is relatively inexpensive and commercially available. In addition, sulfuric acid is also used because SO2 and SO3 may also be present in the flue gas and collected by the catalyst during operation in the power plant facility and when a catalyst is submerged in water or another aqueous solution, the SO2 or SO3 is removed from the catalyst and forms a diluted sulfuric acid solution. However, treating a catalyst with sulfuric acid has disadvantages since the H2SO4 in a diluted aqueous solution also corrodes the steel casings of the catalyst. Corrosion of the catalyst casings may also result in release of water soluble iron compounds that can penetrate the pores of the SCR catalyst, further enhancing the undesired SO2 to SO3 conversion process.
The removal of iron contaminants from a DeNOx catalyst has been described in U.S. Pat. No. 7,569,506 in which the catalyst is placed in a reaction solution comprising an aqueous solution of an inorganic or organic acid with the addition of one or more antioxidants. Inorganic acids, namely hydrochloric acid, phosphorus acid, nitric acid, and, in particular, sulfuric acid, are described. Organic acids, such as relatively strong organic acids, including oxalic acid, citric acid, malonic acid, formic acid, chloroacetic acid, and benzole sulfonic acid were also used. Although the methods described in this reference were effective in removing iron accumulation on the catalyst, the strong acids described in the reference also liberated iron ions from steel substrates and the steel casings of the catalyst. These iron ions can then penetrate the pores of the catalyst, potentially enhancing the undesired SO2 to SO3 conversion.
Thus, there is a need for alternative methods for regeneration of SCR catalysts to remove or minimize contamination by iron compounds and provide optimum DeNOx performance of the catalyst while minimizing or reducing the SO2 to SO3 conversion process within the flue gas stream.